Tuners provide the function of frequency selection in radio and television receivers. Typically, tuners include at least one tuned circuit having inductance and capacitance values selected to resonate at a desired frequency. In certain UHF television tuners, the inductance often comprises a transmission line while the capacitance may be provided by a varactor diode having a capacitance which varies with applied voltage for controlling the resonant frequency of the tuned circuit.
Radio frequency (R.F.) signals customarily are applied to a tuner by capacitive or inductive coupling. Both coupling methods facilitate D.C. isolation of an antenna from the tuner so as to avoid a potential shock hazard in receivers of a type in which the chassis may be coupled to one side of an A.C. power line. An example of the use of capacitive coupling of a balanced 300 Ohm antenna line to a VHF tuner is described by Valdettaro in U.S. Pat. No. 3,938,046 which issued Feb. 10, 1986. To prevent damage to the coupling capacitors by static charge build-up on the antenna line, one conductor is coupled to ground via a high valued (3.3 Meg-Ohm) static discharge resistor and is coupled to the other conductor via a moderate valued (a few thousand Ohms) line bridging resistor.
An example of magnetic coupling of a balanced antenna lines to a UHF tuner is described in Reynolds in U.S. Pat. No. 3,702,958 which issued Nov. 14, 1972. In the Reynolds tuner, an antenna coupling coil is supported by its leads so as to be in axial alignment with the tuner inductor. The coil leads are fastened directly to a pair of antenna terminals mounted on an insulating member fastened to the outside of the tuner housing. Static charge build-up is discharged by means of a bleeder resistor connected between one of the antenna terminals and the tuner housing.
A further example of a D.C. isolated magnetically coupled UHF tuner is described by Suzuki in U.S. Pat. No. 3,755,764 entitled ANTENNA COIL SUPPORT FOR A TUNER. The Suzuki tuner employs a transmission line inductor in its resonator circuit. For proper coupling to a transmission line inductor, it is important that the plane of the antenna coil be normal to the plane of the transmission line and that the coil major axis be aligned with the major axis of the transmission line. Suzuki achieves alignment by attaching the ends of a one-turn coil to an antenna terminal block that extends through an aperture in the side wall of the tuner, the aperture being aligned with the major axis of the transmission line. The free end of the one-turn coil is then secured in an upright position over the transmission line by means of a U-shaped clamp. The antenna connector block has terminals that are normal to the plane of the coil thus necessitating a one-half turn twist in the coil ends to ensure that the plane of the coil is normal to the transmission line over most of the length of the coil.